210 bulletin: museum of comparative zoology. 



pro-otic myomere), Hoffmann ('94, '96, '97, '99, :00), von 



Kupffer ('94, p. 58 : probably a lateral body muscle, but deri- 

 vation from a velar muscle not entirely disproved ; '95, 

 pp. 36, 72: m. rect. ext. of Petromyzontes identical with, tbat 

 of Gnathostomi), Sewertzoff ('95, '98-99, formed from one 

 somite in sharks, from at least two in Torpedo), Kolliker ('96), 

 Neal ('96, m. rect. ext. formed from four metameres). 

 b. Based on anatomy in adult: same as oculomotor nerve (I. 1. b.). 

 2. Lateral-motor nerve (visceral muscle). 



a. Based on ontogeny : Balfour ('78), Marshall ('81), Dohrn ('85, 



p. 447: m. rect. ext. formerly a gill muscle), von Kupffer ('94, 

 p. 58 : derivation from the velum not disproved, though prob- 

 ably from a lateral body muscle). 



b. Based on histology of external rectus muscle : Stannius ('51), 



Langerhans ('73). Schneider ('79) maintains the contrary 

 [ventral motor nerve]. 



Summary of Results. 

 I. Oculomotor Nerve. 



1. The oculomotor nerve of the chick first appears during the third 

 day of incubation as a small bundle of peripheral processes of neuroblasts 

 on either side of the median plane. These neuroblasts are grouped to 

 form the oculomotor nidulus, which is situated in the ventral wall of the 

 mid-brain near the median plane. 



2. Bounded medullary cells (indifferent cells of Schaper) migrate 

 from the neural wall into the root of the developing oculomotor nerve. 

 During the peripheral growth of the neuraxons, these escaped medullary 

 elements continue their migrations, and become distributed along the 

 nerve as "accompanying" cells. In vom-Eath preparations the "ac- 

 companying" cells exhibit staining qualities which differentiate them 

 from the neighboring cells of the mesenchyme. 



3. The majority of the " accompanying " cells become elongated, and 

 remain during development closely applied to the neuraxons. They are 

 recognizable in later stages as the cells from which the sheaths of 

 Schwann are derived. The sheaths of Schwann are consequently, like 

 the other parts of the oculomotor neurons, ectodermal in origin. A 

 small number of the " accompanying " cells retain their rounded form, 

 and through increase in numbers by mitotic division give rise to a 

 cluster of cells near the distal end of the oculomotor nerve. In this 

 way the fundament of the ciliary ganglion is formed, many of the cells 

 of the enlarged extremity of the nerve developing into ganglion cells. 



